The U.S. Department of State has issued a Europe Travel Alert warning American citizens of a “continued threat of terrorist attacks throughout Europe.”

“Recent, widely-reported incidents in France, Russia, Sweden, and the United Kingdom demonstrate that the Islamic State of Iraq and ash-Sham (ISIS or Da’esh), al-Qa’ida, and their affiliates have the ability to plan and execute terrorist attacks in Europe,” states the Travel Alert, which is dated May 1.

The newest alert follows a State Department “Worldwide Caution” issued in March 2017, which warned U.S. citizens that they were a target group for terrorist activities throughout the world. This latest alert focuses specifically on Europe and the potential for further attacks, following a number of recent incidents that give cause for alarm.

U.S. citizens should be alert to the possibility “that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning,” the Alert states, and terrorists may employ a variety of tactics, “including firearms, explosives, using vehicles as ramming devices, and sharp-edged weapons that are difficult to detect prior to an attack.”

The State Department advises U.S. citizens abroad to keep up on local security messages in European cities, by periodically checking the website of their U.S. embassy or consulate.

“U.S. citizens should exercise additional vigilance in these and similar locations,” the Alert adds, “in particular during the upcoming summer travel season when large crowds may be common.”

The State Department assures American citizens that it is working closely with European partners and allies on the threat from international terrorism.

“Information is routinely shared between the United States and our key partners to disrupt terrorist plotting, identify and take action against potential operatives, and strengthen our defenses against potential threats,” it states.

Earlier this year, a U.S. official told CNN that the terror threat in Europe was as high as it’s ever been, adding that as many as 1,900 foreign fighters have returned to Europe after spending time in Iraq and Syria.

While some of these returned disenchanted, others are still committed to terror groups like the Islamic State, the official said.